README: Move to Markdown, rewrite introduction

Move the README file to Markdown, and update it to attempt to explain the current status and use of Weston.

The first sections are user-facing, so they can quickly understand what Weston is, what it does, what it doesn't do, and how to go about using it. The following sections on libweston and for distribution packagers are left intact, but should probably be moved to separate documents.

This includes a screenshot of Weston running weston-terminal, Chrome and simple-egl, which was taken by myself and subject to the same licensing terms as the rest of the tree.